[stdlib] Fix a data race in Array initialization code

It is safe to only reset the counter when the count is non-zero. We always initialize the count to zero. This race has been reported by TSan.

Author: AnnaZaks

stdlib/public/core/Arrays.swift.gyb

@@ -539,8 +539,10 @@ extension ${Self} : RangeReplaceableCollection, _ArrayProtocol {
       // unnecessary uniqueness check. We disable inlining here to curb code
       // growth.
       _buffer = ${Self}._allocateBufferUninitialized(minimumCapacity: count)
+      _buffer.count = count
     }
-    _buffer.count = count
+    // Can’t store count here because the buffer might be pointing to the
+    // shared empty array.
   }
 
   /// Entry point for `Array` literal construction; builds and returns